The present invention relates to a new process for the preparation of aniline compounds and to their use as intermediates in the preparation of herbicides of the isobenzofuranone type.
xe2x80x9cBerichte der deutschen chemischen Gesellschaftxe2x80x9d, 1893, page 2060, already makes mention of the reduction of nitrobenzene to aniline using hydrazine hydrate in alcoholic solution. xe2x80x9cJournal fxc3xcr praktische Chemiexe2x80x9d, 1896, pages 433 to 447, discloses the use of phenylhydrazine in the reduction of nitrobenzenes to corresponding anilines. A further description of the action of hydrazine on nitro- and chloronitro-benzenes is found in xe2x80x9cJournal fxc3xcr praktische Chemiexe2x80x9d, 1925, pages 277 to 284. The reactions are carried out without exception at elevated temperature, in some instances proceeding in alcoholic solution.
It has now been found, surprisingly, that the reduction of nitrobenzenes to anilines can be significantly improved if, instead of alcohol, an aqueous base is used as solvent.
The present invention accordingly relates to a process for the preparation of aniline compounds of formula: 
wherein
n is an integer from 1 to 5 and
R is hydrogen, alkyl, hydroxyalkyl, alkylamino, dialkylamino, alkenyl, alkynyl, alkoxy, alkylthio, phenyl, naphthyl, phenoxy, phenylthio, halogen, amino, hydroxy, mercapto, carboxyl, sulfo, nitro, nitroso, hydroxylamino or heterocyclyl, by reacting nitro compounds of formula 
wherein n and R are as defined, with hydrazine at elevated temperature in the presence of a solvent and isolating the compounds of formula I, in which process an aqueous base is used as solvent.
In the compounds of formula I, n is preferably an integer from 1 to 3. n is especially 2. Those compounds of formula I which have a carboxyl or sulfo group or a salt thereof have been found to be especially suitable and especially valuable. In particular, the compound of formula I which has a carboxyl group in the ortho position and a hydroxyalkyl group, especially a CH3CH(OH) group, in the meta position (3 position) has proved to be especially advantageous. The nitro compound of formula II corresponding to that compound is present in aqueous solution in a pH-dependent hydrolysis equilibrium with the corresponding closed lactone form, 3-methyl-7-nitro-3H-isobenzofuran-1-one. The latter compound, which can also be obtained, for example, by reduction of the 2-nitro-6-acetylbenzoic acid known from Horii et al., Yakugaku Zasshi, 1954, 466 and Baker et al., J. Org. Chem. 1952, 17, 164 using sodium borohydride, is new and the present invention relates also thereto. That compound too can be advantageously used in the preparation of herbicides of the isobenzofuranone type.
In general, compounds of formulae I and II that have, adjacent to one another, substituents capable of together forming a (fused-on) ring, for example a carboxyl group in the ortho position and a hydroxyalkyl group in the meta position, are present in aqueous solution in a pH-dependent equilibrium with the corresponding closed form, e.g. the lactone form, the 5-membered ring lactone form, for example in 7-amino-3-methyl-3H-isobenzofuran-1-one, being especially readily formed in acid solution. As a rule, the tendency towards ring formation decreases with increasing ring size. 6- and 7-membered rings generally form less readily than the 5-membered rings.
The process according to the invention therefore also encompasses the preparation of those closed forms of the compounds of formula I wherein two substituents R have formed a fused-on ring.
The alkyl radicals appearing in the definitions of R preferably contain from 1 to 4 carbon atoms and are, for example, methyl, ethyl, propyl and butyl and branched isomers thereof. Preferred alkoxy, alkylthio and hydroxyalkyl radicals are derived from the mentioned alkyl radicals. Alkenyl and alkynyl radicals R preferably have from 2 to 4 carbon atoms and are, for example, ethenyl, propenyl, ethynyl, propynyl and propenyl and branched isomers thereof, and butenyl and butynyl and branched and di-unsaturated isomers thereof. The terms hydroxy (xe2x80x94OH), mercapto (xe2x80x94SH) and sulfo (xe2x80x94SO3H) and carboxyl (xe2x80x94CO2H) also include in each case the salt form thereof, for example alkali metal, alkaline earth metal and ammonium salts. Heterocyclyl is understood to mean preferably from 4- to 8-membered, saturated or unsaturated rings that contain at least one hetero atom selected from nitrogen, sulfur and oxygen. Examples thereof are pyridyl, furanyl, thiofuranyl, oxetanyl, thiazinyl, morpholinyl, piperazinyl, pyridazinyl, pyrazinyl, thiopyranyl, pyrazolyl, pyrimidinyl, triazinyl, isofuranyl, pyranyl, piperidyl, picolinyl, thiadiazolinyl, thietanyl, triazolyl, oxazolanyl, thiolanyl, azepinyl, thiazolyl, isothiazolyl, imidazolyl or pyrrolyl.
Hydrazine may be used as such or, preferably, in the form of its hydrate. Preference is given to the use of from 1.4 to 3 mol, especially from 1.6 to 2 mol, of hydrazine compound per mol of nitro compound.
The phrase xe2x80x98elevated temperaturexe2x80x99 preferably denotes a temperature range of from 30 to 150xc2x0 C. It is especially preferable to proceed in a range of from 70 to 100xc2x0 C., as reaction temperatures of more than 100xc2x0 C. require the use of pressure.
A suitable aqueous base is especially an aqueous solution of an alkali metal hydroxide, alkaline earth metal hydroxide, alkali metal carbonate or alkaline earth metal carbonate. Organic amines, for example alkylenediamines, urotropine and quinuclidine, are also suitable. Preference is given to the use of from 0.5 to 5 mol, especially from 1 to 2 mol, of base per mol of nitro compound. If the nitro compound of formula II already contains acid groups as substituents, an additional mole of base is required for each acid group.
The process according to the invention has the major advantage that it can be carried out on a large industrial scale. The procedure is generally that the compound of formula II is introduced into water and base is added. After heating the resulting mixture to the desired reaction temperature, hydrazine or hydrazine hydrate is metered in.
The process according to the invention can be carried out either continuously or intermittently (discontinuously, batch-wise), with preference being given to intermittent operation. Both the intermittent and the continuous reaction procedures are carried out preferably in a stirred vessel or a stirred vessel cascade.
Isolation of the aniline compounds is dependent upon the structure and nature of the compound and is carried out where appropriate after acidifying the reaction mixture with, for example, hydrochloric acid to a pH in the range from 1 to 9, especially from 5 to 8, either by crystallisation or, if the product is liquid at room temperature, by extraction with an organic solvent, for example toluene.
The yields of isolated aniline compound are generally in the range from 80 to 100%. The chemical yield in the reaction mixture is usually more than 97%.
The process according to the invention has the following advantages over the process of the prior art:
it can be performed on a large industrial scale
the reaction can to a large extent be controlled by means of metered additions, which is advantageous from the aspect of safety
compared with ethanol as solvent, the reaction proceeds quickly and very selectively
when starting from compounds of formula II, especially those wherein R is sulfo or carboxyl or a corresponding salt, in the form of an aqueous solution, as may be the case, for example, in the continuous process, those compounds can be reduced directly by the addition of base together with hydrazine
it results in products in yields of up to 100%
it can be performed in a multi-purpose apparatus.
The aniline compounds of formula I prepared according to the invention are used, in particular, as intermediates in the preparation of herbicides of the isobenzofuranone type, which are described, for example, in U.S. Pat. No. 5,332,717 and U.S. Pat. No. 5,428,002.
The Examples that follow illustrate the invention further.